This invention relates to a method and apparatus for counting the blood platelets in blood.
The counting of blood platelets has been conventionally achieved by the plural platelet plasma method (PRP method or Bull's correction) that requires the preliminary treatment of a sample or the visual observation method that needs the preparation of a specimen. According to the PRP method, a blood sample in a capillary tube is let stand or centrifuged to settle red blood cells and white blood cells in layers and recover the layers of platelets and plasma which are passed through a corpuscle counter to count platelets one by one. In this method, the suspension of blood platelets is concentrated to separate red blood cells and white blood cells, the count of platelets is corrected by a corrective value related to a separately determined hematocrit of blood to convert the count to the number of platelets in a unit volume of blood. The PRP method achieves relatively high accuracy without requiring skill in preparing a sample, but on the other hand, preparing layers of platelets and plasma requires a lot of time and effort, the slightest presence of red blood cells can cause a great error, and the process of conversion by correction is cumbersome. For these reasons, it is difficult to perform the PRP method automatically.
The visual observation method depends on direct counting of platelets in a blood preparation observed under a microscope. The disadvantages of this method are non-uniformity of platelet distribution in the preparation, a lot of skill needed to make the preparation, and platelets easily destroyed by being deposited on the glass to which the blood sample is fixed.
Therefore, one object of this invention is to provide a method and apparatus for achieving automatic and accurate counting of blood platelets on the basis of simple dilution of a blood sample as is required in the counting of red blood cells.
Another object of the invention is to provide an automatic platelet counting method that involves ony the RBC count as a parameter for correcting the count of platelets and which outputs the final result through a relatively simple arithmetic operation.
A further object of the invention is to provide an automatic platelet counting apparatus that can be manufactured at relatively low cost because of the relative simplicity of the platelet counting principle on which it operates.
Still another object of the invention is to provide an even simpler and more accurate method and apparatus for blood platelet counting which is particularly adapted for clinical screening as a preliminary examination.
The automatic counting technique of this invention can be incorporated in a conventional automatic blood analyzer that permits simultaneous counting of such parameters as RBC count, WBC count, hemoglobin value and hematocrit. By so doing, the number of blood platelets can be counted simultaneously with the determination of other blood parameters.
When a diluted suspension of whole blood is immediately subjected to the counting of platelets, the suspension is sucked into the detector and corpuscles other than platelets that are suspended near a very small opening for detecting a change in electrical impedance that occurs upon a corpuscle passing through the opening enter that opening by "entrainment" to produce an unwanted pulse signal and cause an error. The conventional technique to prevent the entrance of unwanted corpuscles has been focused on the improvement of the construction of the detecting area, for example, all corpuscles such as red blood cells and platelets are sucked through an opening in a double-walled detector while a clean diluted sample is being supplied from around the opening. This improvement enables blood platelets to be counted with satisfactory accuracy by simply discriminating the peaks of individual electrical pulses generated by corpuscles which pass through the opening, but the improvement has never been implemented in a commercial analyzer because it is difficult and costs high to supply a clean dilution free of any foreign matter and unwanted corpuscle and to fabricate a very small double-walled detection area.
The basic principle of this invention is the same technique that detects the passage of a corpuscle by a change in electrical impedance, except that it corrects the count of platelets by the number of electrical output pulses due to red blood cells (including white blood cells) on the basis of the statistically calculated increase in the number of pulses caused by the entrainment of red blood cells (including white blood cells to pass through the detection area).